An internal combustion engine equipped with an EGR system is well known. In the EGR system, a part of exhaust gas emitted from the engine is recirculated into an intake passage through an EGR passage. The quantity of the recirculated exhaust gas is controlled by an EGR valve. U.S. Pat. No. 7,269,497-B2 shows an EGR system in which the quantity of exhaust gas passing through the EGR valve is estimated by means of an EGR valve model and this estimated quantity is filtered to estimate a quantity of the exhaust gas flowing into a cylinder. An opening degree of the EGR valve is feedback controlled in such a manner that the quantity of the exhaust gas flowing into the cylinder agrees with a target value.
JP-2008-101626A shows, in detail, a model for estimating the quantity of the exhaust gas flowing into a cylinder. The quantity of the exhaust gas passing through the EGR valve is computed by means of an EGR valve model which simulates a behavior of the exhaust gas passing through the EGR valve. When the engine is stably running, it is deemed that the quantity of the exhaust gas passing through the EGR valve is equal to the quantity of the exhaust gas flowing into a cylinder. When the engine is in a transitional running condition, the quantity of the exhaust gas flowing into the cylinder is computed based on the quantity of the exhaust gas passing through the EGR valve by means of a EGR diffusion model (time delay+first order lag) which simulates a behavior of the exhaust gas flowing in an intake pipe toward an intake port of the engine.
It should be noted that the opening degree of the EGR valve is feedback controlled in such a manner that the quantity of the exhaust gas passing through the EGR valve agrees with a target value.
In order to improve fuel economy of the engine equipped with an EGR system, it is necessary to control the quantity of the recirculated exhaust gas flowing into a cylinder (or an EGR ratio) according to an engine driving condition so that gas including exhaust gas is efficiently combusted in a cylinder. Thus, it is necessary to feedback control the opening degree of the EGR valve in such a manner that the quantity of the exhaust gas flowing into a cylinder (or an EGR ratio) agrees with the target value with high accuracy.
However, in the EGR system shown in the above two patent documents, it became apparent that a control accuracy of the quantity of the exhaust gas flowing into a cylinder does not satisfy the required accuracy to improve the fuel economy. This reason can be considered as follows:
(1) In the conventional system, it is difficult to obtain high control accuracy and high control stability in the feedback control of an EGR valve.
(2) An accuracy of the model which estimates the quantity of the exhaust gas flowing into a cylinder is not high.